FlatterTest.java:44-46: Enable several tests in this...

[0pdd]

The puzzle 23-0dd3745a from #23 has to be resolved:

flatty/src/test/java/org/objectionary/FlatterTest.java

Lines 44 to 46 in 95dace5

	* @todo #23:30min Enable several tests in this FlatterTest class.
	* Since they are failed due to NPE, we need to fix them. The tests
	* are disabled for now.

The puzzle was created by @Graur on 03-Jul-23.

Estimate: 30 minutes, role: DEV.

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